This invention relates to the field of document printing/copying. More particularly, it relates to improvements in apparatus for precisely and repeatedly positioning an electro-optical printhead or xe2x80x9coptical writer,xe2x80x9d e.g. a linear array of light-emitting diodes, relative to a reusable image-recording drum in a document printer/copier, that is, an electrophotographic printer and/or copier, to enable, for example, removal, servicing and replacement of the optical writer and/or image-recording drum without altering a desired positional relationship between the optical writer and image-recording drum inside the document printer/copier.
The above-referenced U.S. patent application Ser. No. 09/474,352 discloses an electrophotographic document printer/copier in which the photosensitive recording element comprises a photoconductive drum assembly having structure for precisely positioning the various image-processing stations relative to the drum""s photoconductive surface. The drum assembly generally comprises a photoconductive drum having axles extending from opposite ends thereof along an intended axis of drum rotation, and a pair of drum-support members, sometimes referred to as xe2x80x9cspiders,xe2x80x9d that support the drum for rotation. Each of the drum-support members has a centrally located bearing for rotatably supporting a drum axle, and a plurality of mechanical fiducials (in the form of rounded buttons or xe2x80x9cbulletsxe2x80x9d) extending in an outward direction relative to the drum""s axis of rotation. Each of these fiducials is adapted to engage and mate with a V-notched block or the like associated with one of the image-processing stations (e.g., the primary charger station or toning station) as these stations are moved from a standby position substantially spaced from the drum surface towards an operative position closely spaced from or actually touching the drum surface. When the respective fiducials of the drum assembly have engaged and become seated in the V-blocks of the processing stations, the operative elements of the processing stations (e.g., the corona wire(s) of the primary charging station, or the development brush of the toning station) will have become precisely spaced parallel to, and/or exert substantially uniform pressure on, the drum surface over the entire width of the drum.
In use, the above-described drum assembly is mounted between a pair of parallel and vertically extending xe2x80x9cmechxe2x80x9d plates comprising the front and rear walls of the printer frame. An opening in the front plate (as viewed from the front of the printer) enables the drum assembly, as well as most of the image-processing stations, to enter the region between the plates. A pair of parallel guide channels, extending between the front and rear walls of the printer frame, engage a pair of ears on the drum-support members and serves to guide the drum assembly to its operative position from a position outside the printer frame. Four dowel pins, two on each frame plate, engage holes in the drum-support members and serve to precisely locate the drum assembly in the printer frame.
Of the different image-processing stations in an electrophotographic printer or copier, the image exposure station is one that requires exceptionally precise positioning relative to the surface of the recording drum. Unless this station is precisely located relative to the nominal position required to produce optimum focus on the photoconductive surface of the drum, the ultimate image on the image-receiver sheet will be noticeably degraded. Ideally, its position within the printer frame should be precisely set by the manufacturer and left undisturbed during the lifetime of the printer. In one respect, this goal is realizable in that the expected lifetime of conventional solid-state printheads is relatively long, far exceeding that of the photoconductive drum assembly and the other image processing stations. However, in printers of the type described above, the printhead must be movably mounted for movement toward and away from the imaging drum to enable the drum assembly to be serviced or replaced. Each time the printhead is moved away from its optimal focus position, there is a chance that it will not be returned to its optimal position. Further, in many systems for positioning the image-processing stations relative to the photoconductive drum surface in a document printer/copier, the exact position of each station will depend on the results of xe2x80x9cstackedxe2x80x9d tolerances. For example, the final position of the printhead may depend on the respective positions of a series of mechanical fiducials, each having nominal position, within a certain tolerance range, that depends on the placement of other fiducials. Since these tolerances are additive, i.e., stacked atop each other, it is desirable to minimize the number of related fiducials required to position an image-processing station.
In view of the foregoing discussion, an object of this invention is to reduce the afore-noted tolerance stacking problem associated with prior art printers insofar as it relates to the positioning of an optical writer relative to the surface of a photosensitive recording element.
Another object of this invention is to obviate any need for retracting the optical writer from its operative position in a printer of the type described in order to remove the drum assembly.
According to the present invention, the photosensitive drum assembly and the optical writer used in an electrophotographic printer/copier of the type described share the same mechanical fiducials for positioning these elements within the printer/copier frame or housing. More specifically, two of the mounting pins used to position and support the drum-support members (spiders) of the drum assembly in the printer frame are also used to position the optical writer. Preferably, the point at which each drum-support member contacts the mounting pins is closely spaced, axially speaking, from the point at which the optical writer contacts the mounting pins.
As a result of this mounting scheme, the afore-mentioned tolerance stacking is reduced since the position of the optical writer is not determined by a fiducial carried by the drum-support members, but rather by the very same fiducial that serves to locate the drum-support member in the printer frame. Also, since the optical writer does not contact the drum-support members, there is no need to retract the writer from the drum-support members in order to free-up the drum assembly for removal from the printer. This results in a saving of parts, since here is no need to provide a retractable mount for the writer, and minimizes the need to flex electronic cables and cooling is lines connected to the optical writer, thereby improving reliability Another advantage of this mounting scheme is that, since the optical writer is mounted within the printer frame in a manner that is totally independent of the drum assembly, the optical writer can be removed from the printer without any mechanism for initially retracting the writer from contact with any portion of the drum assembly.
Thus, according to the present invention, an electrophotographic printer/copier comprises: (a) a frame; (b) a drum assembly comprising (i) a drum having a photoconductive surface upon which a plurality of work stations operate to produce an image; and (ii) a pair of drum-support members for rotatably supporting said drum for rotation about a drum axis; (c) an optical writer for projecting image information onto said photosensitive surface; and (d) a mechanical fiducial mounted on said frame, said drum assembly and said optical writer sharing said mechanical fiducial for positioning said drum assembly and said optical writer in said frame to achieve a desired positional relationship between the optical writer and the photoconductive surface of said drum. Preferably, the frame comprises a pair of spaced frame plates, and the mechanical fiducial comprises a pair of dowel pins, one of such pins extending outwardly from each of the frame plates at locations to be engaged by opposite ends of the drum assembly and optical writer.
The invention and its technical advantageous effects will be better appreciated from the ensuing detailed description of a preferred embodiment, reference being made to the accompanying drawings in which like reference characters denote like or functionally similar parts.